This invention relates to a drum brake device in which a single shoe extension device activated by the service brake can yield a stable braking force equivalent to that of a two-leading (2L) type of drum brake device.
Drum brake devices which employ a single shoe extension device activated by the service brake are generally of the leading-trailing (LT) or duo-servo (DS) type. However, although the LT drum brake provides stable braking, the braking force is low, being particularly insufficient for use on trucks. On the other hand, a DS drum brake presents uncertainties as to stability upon braking because a sudden increase of the braking effect when the coefficient of friction being higher can induce early locking of the brake, or can easily create an imbalance in the braking force between the left and right wheels. Two drum brake devices known to provide relatively good braking power and stable braking effect are the two-leading (2L) and dual-two-leading (D2L) types, both of which are equipped with two shoe extension devices, such as a wheel cylinder, positioned symmetrically about the axle.
A 2L or D2L drum brake device each has the following issues to be considered. Incorporation of a parking brake device is difficult in part because of layout restrictions and increased intricacy of structure. A separate transmission brake device can be used, but that increases the weight and the cost of the vehicle.
The presence of two shoe extension devices makes it difficult to design the drum brake with just one automatic shoe clearance adjustment mechanism which can automatically adjust the gap between the brake drum and the brake shoes.
The presence of two shoe extension devices not only makes it difficult to arrange the brake fluid lines, but also increases the cost of the device.
A drum brake device which can provide a braking action equivalent to a 2L type has been disclosed in Japanese Utility Model Publication No. Showa 50 (1975)-38704. In this device, a shoe extension device is set between the one pair of adjacent ends of two facing brake shoes, and a crank mechanism, which functions together with the back plate, is set between the other pair of adjacent ends. This device has certain problems.
The addition of an automatic shoe clearance adjustment device to the shoe extension device increases its cost. In addition, the brake shoes at the shoe extension device side separate as the lining wears. Since an anchor to bear the braking force of the brake shoe is mounted in the device, it is difficult to install a parking brake device.
A spring is stretched on a lever so that the crank mechanism retains its neutral state when there is no braking. However, when the shoe clearance is being adjusted, the crank mechanism is supported on the back plate by only one fulcrum, whereby the spring forces acting on the mechanism become easily imbalanced. Unequal spring forces acting on the crank mechanism can cause the brake shoes to drag when the vehicle is in motion.
Since the large quantity of components in the crank mechanism, the large number of pivoting components, and the large number of points bearing the braking force, reduces the durability of the drum brake device, the heavier and stronger components must be used. Also, the layout is extremely complex, with the crank mechanism being arranged in a crowded space.